Leak detector for evacuated systems



Oct. 17, 1950 H. NELSON LEAK nsmc'ron is'oa EVACUATED sYs'mls Filedlarch 7, 1947 lllllllililtatlll I!!! Juvcntor .zfmzer 114 250 in Y WGttomeg Patented Oct. 17, 1950 I LEAK DETECTOR FOR EVACUATED SYSTEMSHerbert Nelson, Bloomfield, N. J assignor to Radio Corporation ofAmerica, a corporation of Delaware Application March 7, 1947, Serial No.7 32,972

12 Claims. 1

My invention relates to the detectionand location of air leaks inevacuated systems and devices, and more particularly to animprovedhydrogen ionization gauge incorporating an automaticallyoperated sensitivity control.

In my co-pending application, Serial No. 575,- 447, filed January 31,1945, now abandoned, I have disclosed an effective apparatus fordetecting air leaks in exhaust systems and evacuated devices by means ofan ionization gauge which comprises an evacuated envelope separated froma the exhaust system or evacuated device by a thin palladium sheet. Whenheated by a filament the palladium sheet becomes permeable to hydrogen.The exhaust system or evacuated device to. be tested is explored orprobed with hydrogen. The presence of a leak will result in an increasein the partial pressure due to hydrogen within the exhaust system or theevacuated device regardless of the pressure of the other gases present.A flow of hydrogen will then take place through the hot palladium intothe low pressure ionization gauge envelope. This increase of hydrogenwithin the gauge envelope causes adetectable change in the positive ioncurrent flow through the tube indicating the leak in the exhaust system.

In normal operation of the leak detecting equipment described in myabove cited co-pending application, lack of sensitivity of the palladiumsheet occurs due to the adsorption of oxyprovide an improved device forthe detection of air leaks in evacuated systems.

It is a further object of my invention to provide a .leak detector whosesensitivity is automatically maintained during operation.

It is a further object of my invention to provide an ionization gaugewhose sensitivity is not gen on the surface of the palladium sheet,which in turn prevents the passage of hydrogen into the envelope of theionization gauge. Another condition producing low sensitivity of thepalladium sheet resulted from the presence of oil vapors in the exhaustsystem in the region of the palladium sheet. The oil tends to dissociatewhen in contact with the hot palladium to produce hydrogen in quantitieslarge enough to make the detection of small leaks impossible.Furthermore, though gauges of the type described in my above citedco-pending application have been found useful in practice, they havebeen found susceptible to damage during use. Thus, the. accidentalentrance of air into the exhaust system to which the gauge is attachedat a time when the heater for the palladium is incandescent causes theevaporation of oxides onto the palladium and a consequent severedecrease in the hydrogen permeability of the palladium. A similardecrease in sensitivity of the gauge occurs with time during normal useof the devicedue to the accumulation on the palladium of materialevaporated from the adjacent heater filament.

It is, therefore, an object of my invention to impaired by a simple andrugged construction.

Itv is also an object of my invention to provide an ionization gaugewhich is not susceptible to damage due to exposure and oxygen duringoperation.

It is also an object of my invention to provide an ionization gaugewhose sensitivity is readily maintained inthe presence of oil vapors.

The novel featureswhich I believe to becharacterlstic of my inventionare set forth with particularity in the appended claims, but theinvention itself will best beunderstood by reference to the followingdescription taken in connection with the accompanying drawing, in which:

Fig. 1 is a sectional view of a hydrogen ionization gauge tube,according to my invention; and

Fig. 2 graphically discloses a hydrogen leak detector apparatus andrelated control circuits, according to my invention.

The ionization gauge tube 24 disclosed in Fig. 1 is an improved deviceover that disclosed in my co-pending application, cited above. The gauge24 of Fig. 1 comprises preferably an exhausted metal envelope 30 closedat one end by a glass header 3|. An anode electrode 33 comprises theenclosed end of a metal tube 32 sealed through the wall of the metalenvelope 30. Sealed across the end of the enclosed anode portion 33 oftube 32 is a palladium cup at. Mounted within the metal envelope 30 is acathode cylinder 34 closed at 31. The top portion 31 of the cathodecylinder 34 is coated, preferably, with a mixture of the oxides ofbarium and strontium so as to pro- "vide a source of electron emissionwhen the cathcathode cylinder 34 is mounted upon support rods 39 fixedto respective lead rods 4| sealed through the glass header 3|. Likewise,filament 35 is supported by a pair of metal leads 4| sealed through theheader 3|. Coaxially enclosing the cathode cylinder 34 is a positive ioncollector electrode 36. The electrode 36 is, preferably, of a tubularconfiguration'and is mounted upon side rods 42 supported from the glassheader 3|, as indicated.

The envelope 38 or the ionization gauge 24 is evacuated in a well knownmanner and the residual gases are cleaned up by flashing of a gettermaterial, not shown, as is well known in the art. The final pressure ofgas within the tube envelope 38 is in the order of 18- millimeters ofmercury. In operation, an appropriate potential drop is establishedbetween the anode electrode 33 and the cathode cylinder 34. When theactivated surface 31 of the cathode is heated to a sufliciently hightemperature an electron discharge will take place between the anode 33and the cathode 34. The palladium metal cup 38 is positioned by thearrangement shown in Fig. 1 in a manner that the discharge between theanode 33 and the cathode 34 will maintain the temperature of thepalladium metal 38 at around 800 C. The palladium cup at thistemperature is permeable to hydrogen gas. If during tube operation thereis any gas present within the tube envelope 38 the electron dischargeestablished between the anode 33 and the cathode 34 will causeionization of the gas to take place. The collector electrode 36 ismaintained at a sufliciently negative potential as to attract thepositive gas ions formed by the discharge. The positive gas ions formedwithinthe tube will be swept out of the interelectrode space andcollected by electrode 36.

Electrode 36 is connected into an appropriate circuit. as will bedescribed more fully below, for establishing a flow of currentresponsive to the amount of gas ionization within the tube. It theamount of gas within the ionization tube 38 is increased, obviously, agreater amount of gas ionization will takeplace and an increase in thegas ions present within the tube will result in an increased flowofcurrent in the related circuit.

The ionization tube shown in Fig. 1 is an improvement over the'corresponding ionization gauge disclosed in my above cited co-pendingapplication. This improved structure of Fig. 1 is of a simplifiedconstruction. The metal envelope 38 may be of any desired design and Ihave found that a conventional metal envelope used for discharge tubesmay be used. Furthermore, with this envelope 38, the arrangement andmounting of the cathode electrode 34 as well as the tubular collectorelectrode 36 may be done in a conventional manner from a glass headerstem 3|. The type of tubular anode electrode 33 shown in Fig. 1

and the arrangement of the palladium metal cup 38 eliminates the need ofan additional heating filament to maintain the palladium metal 38 at thenecessary temperature of around 800 C. This arrangement then not onlyeliminates the palladium heater but also the palladium heater ciringapplication.

As shown in Figure 2, ionization gauge 24 is connected to appropriatecircuits for its operation. Anode tube 32 is connected directly to apositive terminal 45 of a direct current power supply while the cathodeelectrode 34 is connected as indicated to a ground terminal 46. Thenegative terminal 41 of the direct current power supply establishes thepotential of ground by being connected thereto. This arrangementestablishes a potential difference between the anode electrode 33 or theionization tube 24 and its cathode electrode 34. This potentialdifference is preferably around 180 volts D. C. which is that maintainedbetween terminals 41 and 45 of the direct current power supply. Avoltage divider resistance 48 is connected between the power supplyterminals 45 and 41. The collector electrode 36 of the ionization gauge24 is connected by a conductor 58 in series with a battery 52 and aresistance 54 to ground at 56. The battery 52, as is indicated, has itspositive terminal connected to the resistance 54. The voltage or battery52 is, preferably, around 22V2 volts so as to maintain the collectorelectrode 36 at a low negative potential relative to cathode electrode34. During the operation of the ionization gauge 24, the dischargemaintained between the cathode 34 and anode 32 will cause gas ionizationto take place within the interelectrode space of the tube 24. Positiveions formed will be swept up by the negative electrode 36. This willresult in a current flow in the circuit between terminal 56 and thecollector electrode 36.

An arrangement is provided for amplifying the positive ion current flowestablished between terminal 56 and the collector electrode 36. Thisamplification may be provided by a D. C. amplifier circuit 51 comprisingan amplifier tube 58 having a control grid 68 connected to a terminalpoint 62 of battery 52 in the positive ionization current circuit. Thecathode 64 of amplifier tube 58 is maintained at ground potential whilethe anode plate electrode 66 is connected in a plate circuit byconductor 68 in series with a load resistance 18, milliammeter 12, to aterminal point 14 of the voltage divider 48. A screen grid 16 adjacentto the anode electrode 66 is maintained at a constant positive potentialso that the plate current of tube 58 will be proportional to the voltageestablished on the control grid 68. The operation of the device is suchthat an increase in the current flow in the positive ionization circuitof collector 34 will cause a shift in the positive direction of thepotential of terminal point 62 and of the control grid 68, resulting ina proportional increase in the plate current of the amplifier tube 58.This plate current change is indicated by the milliammeter 12. In likemanner, a decrease in the positive ion current will cause a resultingdecrease in the plate current.

In Fig. 2, there is disclosed an arrangement for detecting leaks inevacuated systems and devices. This apparatus comprises principally anexhaust manifold l8 connected through stopcocks 28 and 22 to an exhaustpump which is not shown. Between the stopcocks 28 and 22 is a branchline l2 by which the manifold in may be connected to any type of exhaustsystem, apparatus or device in which it is desirable to detect anexisting leak. For example, as shown in Fig. 2, there is connected tothe exhaust manifold I8 a discharge device |8. The discharge device I8is connected in communication with the exhaust manifold branch 12 by itsexhaust tubulation I 6 through a rubber port l4. The exhaust manifold I8is connected at its other end in communication with the ionization gauge24 by the metal anode tube 32. Between the stopcock 28 and the anodecommunication'tube 32 of the ionization gauge 24 there is provided atrap portion 26 immersed in preferably liquid air within a container 28.In the exhaust manifold I8 there is present at all times hydrocarbonvapors which come from the oils and greases used in the vacuum pump usedand in the stopcocks 28 and 22. The purpose of the liquid air trap 2828is to prevent the diffusion of these hydrocarbon vapors as well asmoisture from the manifold I8 into the region of the palladium barriersheet 38. With the discharge device I8 in place, stopcocks 28 and 22 areopened and the exhaust manifold I8 is connected to any desirable exhaustmeans such as a rotary exhaust pump. The system is exhausted until thepressure of the gas within the device I8 as well as manifold I8 isreduced to a final pressure in the order of millimeters of mercury.

To discover the location of a leak in the dis charge device I8, a hood16 containing hydrogen is placed over the discharge device I8 tocompletely immerse it in a hydrogen atmosphere. If any leak exists inthe evacuated discharge device I8, hydrogen will enter the device andpass by the exhaust tubulation I8 into the exhaust manifold I8. Theentering of hydrogen into the exhausted system from the discharge deviceI8 will cause a. rise in the gas pressure within the manifold I8 andwill result in the passage of hydrogen through the palladium sheet 38into the ionization gauge 24 in order to equalize the hydrogen pressureon both sides of the palladium sheet 38. An increased quantity of gas inthe discharge-device 38 will result in an increase in the amount of gasionization in the tube and a consequent larger flow of current in thecircuit of collector electrode 34. As described above, this increasedpositive ion current flow can be detected by the milliammeter 12 ofcircuit 51.

The detection of the presence of a leak in a discharge device I8 may beperformed with both of the stop cocks 28 and 22 open and while theexhaust pump is running to maintain a low pressure within the device I8and manifold I8. Thus, if the leak is large, an excess of the hydrogengas flowing through the leak into the device I8 will be removed from themanifold I8 and the remainder will be suflicient to be detected by thegauge 24 and milliammeter I2. In the case of a large leak in the deviceI8 and if the stop cock 22 were closed, an excess amount of hydrogenwithin the manifold system I8 would adversely affect the operation ofthe ionization gauge 24. However, if the leak in the device I8 issufficiently small so that it cannot be detected with the manifold I8connected to the pump, then the stop cock 22 to the pump is closed andthe hydrogen hood I6 is maintained about the device I8 until suflicienthydrogen has leaked into the manifold I8 .to cause an increase in thegas ionization within the tube 24 so as to be detected by themilliammeter I2.

The detection of the presence of a leak in the discharge device I8 asdescribed above may be followed by removing the hydrogen hood I6 andprobing for the leak by a sharp hydrogen jet in the suspected areas ofthe discharge de-.

vice I8. When the specific locality of the leak is found by the sharphydrogen jet, its presence will also result in a change in the positiveion current as indicated by the milliammeter I2.

In the operation of an ionization gauge, using a palladium barrier, ithas been found in practice that the sensitivity of the palladiumdecreases due to several different conditions. In the exhaust manifoldsystem, the hydrocarbon vapors derived from the oils and greases used inthe vacuum pump and in the stopcocks used in the exhaust manifold willdiffuse during a period of disuse into the region of the palladium sheetand onto the surface of the palladium. when, under these conditions, theionization gauge is, put into operation, the heated palladium plate willdecompose the hydrocarbon vapors to form of inactivity, maybe burnedout.

a large amount of hydrogen. The presence of ,this large amount ofhydrogen will tend to mask any small increase of the manifold pressuredue to the entrance of hydrogen through a leak in the exhaust-devicebeing tested. Furthermore, the presence of air at different times withinthe the exhaust manifold will cause the formation of oxides upon theexposed surface of the hot palladium sheet. These oxides decrease thesensitivity of the palladium by preventing the passage of hydrogentherethrough.

With my new type of ionization gauge described above relative to Figures1 and 2, I am able to eliminate the above mentioned conditions which areso detrimental to the sensitivity of the ionization gauge. Oil vapors,which may have condensed in tube 32 and on the palladium sheet 38 as theresult of the absence of a cooling agent around the trap 36 durin aperiod I This is done by opening manifold to the atmosphere and by,operating tube 24 in thenormal manner to produce a heating of thepalladium sheet 38 and the neighboring regions of tube 32. About twominutes are allowed for the burning or oxidation of the hydrocarbons inthe region of the palladium. The manifold may then be re-evacuated inpreparation for itsnormal use as described above. What hydrogen gaswhich was evolved from the decomposition of the hydrocarbon vapors incontact with the hot palladium sheet 38 and which passed at that timeinto the ionization gauge 24, is largely pumped back through thepalladium sheet 38 until there remains within the ionization gaugeenvelope38 a pressure of hydrogen in the order of 10". This will equalthe partial pressure of hydrogen remaining in the manifold I8 afterexhaust. When the exhaust manifold I8 has been exhausted as far aspossible by the pump, the stopcock 22 is closed and the detector isready for use.

To both remove oxides previously formed on the palladium and also tomaintain a condition such that oxides are not subsequently formed on thepalladium during operation of the device" due to the presence of oxygenin the manifold I8, I have found it expedient to maintain a small amountof hydrogen within the system in the order of 1 10- millimeters ofmercury. This hydrogen must be continuously replaced since due to getteraction within the gauge 24 and the absorption of hydrogen on the surfaceof the walls of the apparatus, hydrogen that is present is continuouslybeing cleaned up. To provide this constant though small supply ofhydrogen within the exhaust system, I provide a filament 88 mountedwithin the exhaust manifold I8. Filament 88 is prefrably of nichromewire which willnot burn ou in the presence of air. It is maintained at atemperature sufiiciently high to decompose the hydrocarbon vapors whichare always present within the exhaust manifold I8 and thus provide aready source of hydrogen.

Since the manifold I8 is exhausted during operation to a pressure in theorder of 10- millimeters, there is always present some gas within themanifold I 8. Part of this gas is hydrogen supplied f'romthedecomposition of the hydrocarbon vapors by the heated filament 88; Thishydrogen will represent a partial pressure of all the gases within theexhaust system and, as

mentioned above, is preferably maintained in the order of about 1 10".This partial pressure of hydrogen within the exhaust manifold III willdistribute itself throughout the system and pass through the heatedpalladium sheet 38 until the pressure of the hydrogen within envelope 30is equal to that within the manifold l0. The presence of hydrogen withinthe envelope 30 due to this partial hydrogenpressure within the exhaustsystem causes ionization to take place and a resultant current flow inthe positive ion circuit of electrode 34. Since this partial pressure ofhydrogen is constantly reduced as mentioned above by the getter actionwithin the ionization tube 24 as well as by its absorption on the wallsof the system, there will be a gradual decrease in the ionizationcurrent detectable by the milliammeter 62. This necessitates a controlfor filament 80 so that the rate of hydrogen generation can beconstantly maintained equal to the rate of absorption of the hydrogen bythe system.

cathode electrode 88 of control tube 88.

Filament 80 comprises a portion of a circuit 18 which provides a currentfiow for heating the filament 80. A voltage potential is provided incircuit 18 by the secondary 82 of a transformer 84. This voltage may beof any desired amount having the proper relationship to the resistanceof the circuit and the filament 80. In one arrangement, the preferredvoltage provided by the secondary 82 was 120 volts. To control thecurrent fiow through the filament 88 there is provided in the circuit 18a gas control tube 86 having an indirectly heated cathode 88 and ananode 90 connected by a conductor 92 to the transformer secondary 82 andthe filament 80. The

transformer secondary coil 82 establishes a potential difference betweenthe anode 90 and the cathode 88 of the control tube 86. Under properconditions there is a discharge through tube 86 which provides a fiow ofcurrent through the circuit I8 for heating filament 80.

As described above, it is desirable that the generation of hydrogen byfilament 89 be controlled so that the hydrogen generation willapproximately be equal to the hydrogen absorption by the getteringaction of the exhaust system. To automatically control the generation ofhydrogen by filament 80 so as to maintain the partial pressure ofhydrogen in the exhaust manifold I0 at a constant value, the filamentcircuit 18 is coupled to the amplifier circuit 51. The control electrode94 of the control tube 86 is connected by a conductor 96 to a terminal98 of the conductor 68 of circuit 51. Furthermore, the cathode 88 of thecontrol tube 86 is connected by a potentiometer unit 99 and conductor 91to terminal 95 of the circuit 51. Between the terminal points 98 and 95of circuit 51 is the load resistance 10 which is preferably of 10,000ohms. A current flow in the output circuit of the amplifier tube 58 willestablish the terminal point 98 at a more negative potential than theterminal point 95 which will result in the control electrode 94 of thecontrol tube 86 being at a more negative potential than the cathodeelectrode 88. The potentiometer unit 99 is essentially a voltage dividerin which a resistance 93 is connected across a battery 9|. By adjustingthe movable contact 89 of connector 91, the potential bias of cathodeelectrode 88 of control tube 86 may be set to any desired bias relativeto the potential of the control elcctrode 9-4. A screen grid 81 isconnected to a phase shift network 85 which operates in a well knownmanner to permit the control tube 81 to conduct a current proportionalto the changes in potential on control grid 16. From this describedarrangement a change in the potential of control grid 94 will cause aproportional change in the current conduction through tube 88 andfilament circuit 18. The relationship of control grid 84 to the outputcircuit of the amplifier tube 68 is such that an increase in currentfiowthrough the amplifier output circuit will cause the potential of thecontrol electrode 94 to be more negative relative to the The impressionof a more negative voltage upon the control grid 94 will reduce theaverage current flow through the control tube 86. In a like manner, adecrease in the plate current of circuit 51 will result in a morepositive bias being placed on control electrode 94 relative to thecathode potential of electrode 88 resulting in an increase in theaverage current flow through tube 86 and filament circuit 18.

The operation of circuit 18 relative to circuit 61 is such that for acertain amount of gas ionization taking place in the ionization gauge 24there will be a definite amount of current fiow through filament 80. Itfollows then that control circuit I8 operates to maintain a current flowthrough filament of an amount inversely proportional to the hydrogen gaswithin exhaust manifold It. If for any reason the absorption of hydrogenwithin the system is increased to a greater amount than the generationof hydrogen by filament 80, there will be a resulting drop of thepartial pressure within the exhaust manifold It. This will result, asdescribed above, in a decrease of the amplified ionization currentthrough circuit 51, which will result in a corresponding increase incurrent flow through circuit 18 and consequently a greater generation ofhydrogen by filament to restore the hydrogen pressure in manifold iii.In a similar manner, if for any reason there is an increase of thepartial pressure of hydrogen within the exhaust manifold it there willresult an increased ionization current in the circuit 61 and a resultingdecrease of heater filament current in circuit 18. This then reduces therate of hydrogen generation by filament 88 so that the partial pressureof hydrogen within the exhaust manifold I0 is restored to its formerpressure.

By the particular automatic filament current control circuit 18, it isthus possible to main tain the partial pressure of hydrogen within theexhaust manifold It at a desired constant value, preferably at apressure in the order of l 10-' millimeters of mercury. The constantpresence of this small amount of hydrogen within the exhaust system l0provides a means for keeping the palladium sheet 38 freefrom theformation of oxides thereon. Elimination of oxides due to the oxidationof the palladium in the presence of air provides a'much greatersensitivity of the palladium sheet 38 so that at all times thepermeability of the palladium is not impaired. Also, in this manner,hydrogen entering the exhaust system H) from a leak in the dischargedevice I8 is rapidly detected by an increase in the amplified ionizationcurrent of circuit 51.

From the above discussion it can be seen that the type of ionizationgauge disclosed at 24 provides an improved device over the prior systemdescribed in my co-pending application cited above. The arrangementdisclosed in Fig. 1 is not harmed by the presence of air or oxygenwithin the exhaust manifold it. As described few minutes.

above, if any oxidation of the palladium'disc 38 takes place it may beeasily removed, by the generation of hydrogen within the system from thefilament 80. Furthermore, the presence of hydrocarbon vapors on or nearthe-sensitive barrier 38 may be quickly eliminated by introducing airinto the system and burning off. the hydrocarbons.

The ionization gauge 24 is also an improved design over the priormodification in the above cited co-pending application. This improvedde-- sign eliminates a separate filament heating elementfor thepalladium window and, furthermore, provides a single envelope 3!! forthe ionization gauge; Furthermore, the elimination of the separateheater element removes the danger of the presence of air in the exhaustsystem during the operation of the ionization gauge. In the formerdesign, disclosed in the above copending application, the heaterfilament for the palladium barrier gave considerable trouble whensuddenly exposed to the atmosphere in itsheated state due to'anyaccidental breakage of the system or inadvertent filling of-the exhaustsystem In by air... The palladium heater filament would oxidize anddeposit the oxides on the adjacent palladium barrier with a resultingloss of sensitivity for the palladium. Furthermore, there was the dangerthat in the presence of air the heated palladium filament would burn outwith aconsequent destruction of the ionization gauge. However,-the abovedescribed system, disclosed in Figs. 1 and 2, provides a simpleconstruction in which the ionization gauge 24 is made of conventionaltube structures withouttively charged collector electrode within saidtubular anode electrode to maintain a gas ionizing electron dischargetherebetween, a palladium sheet sealed across to close said one end ofsaid tubular anode electrode, said palladium sheet positioned inthe pathof said discharge to be heated thereby, whereby said sheet will becomepermeable to hydrogen, means including a negaenvelope for establishing'acurrent fiow responsive to positive hydrogen ions within said en-'velope, and means responsive to said positive hydrogen ion current tomaintain a quantity of hydrogen gas within said manifold, saidresponsive means including a heated filament within said manifold tobreak down hydrocarbon vapors therein.

2. A device for detecting leaks in an evacuated system, said devicecomprising an exhaust manifold adapted to be maintained at a' lowpressure and to be connected in communication with said evacuatedsystem, an evacuated envelope joined to said manifold, a metal sheetpermeable to hydrogen when heated, said metal sheet positioned toseparate the interior of said manifold from the interior of saidenvelope, means including spaced electrodes within said envelope forestablishing a hydrogen gas ionizing discharge therebetween, a firstcircuit including a negatively charged collector electrode within saidenvelope for providing a current responsive to positive gas ions withinsaid envelope, a filament within said manifold to maintain a quantity ofhydrogen gas therein, a second circuit including .said filament forproviding a filament heating current therefor and means responsive tosaid said positive ion current.

in checking graded-metal seals and other types used in both largereceiving and miniature elec tron tubes. The sensitivity of the deviceis such that leaks in the order of 1 10- liter microns 'per sec. ofhydrogen, which formerly were im- I possible or difiicult tolocatebecause of their small size, can now be accurately located in aused to check any glass or metal vacuum systems such as. employed inprocessing equipment, or, for example, vacuum stills, vacuum furnaces,cathode ray tubes, vacuum spectographs, etc.

While certain specific embodiments have been illustrated and described,it will be understood that various changesand modifications may be madetherein without departing from the spirit and scope of the invention.

What I claim as new is:

1. A device for detecting leaks in an evacuated system, said devicecomprising an exhaust manifold adapted to be maintained at a low gaspressure and to be connected in communication with said evacuatedsystem, an evacuated envelope, a tubular member connecting said manifoldin communication with said envelope, one end of said tubularmember'enclosed within said envelope to form an anode electrode, acathode electrode spaced within said envelope from said The leakdetector may also be- 3. A device for detecting leaks in an evacuatedsystem, said device comprising an exhaust manifold adapted to bemaintained at a low gas pressure and to be connected in communicationwith said evacuated system, the low gas pressure within said manifoldincluding a partial pressure'due to the presence of hydrocarbon vapors,an evacuated envelope, means joining said envelope in communication withsaid manifold, a

palladium barrier closing said communication between said envelope andsaid manifold, an anode and cathode electrode spaced within saidenvelope to provide a gas ionizing electron discharge therebetween, saidpalladium barrier positioned to be heated by said electron dischargewhereby said barrier will become permeable to hydrogen, a first circuitincluding a negatively charged collector electrode within said envelopeto establish a current flow responsive to'positive ions within saidenvelope, a filament within said manifold to break down said hydrocarbonvapors and maintain apartial pressure of hydrogen within said manifold,a second circuit including said filament to provide a filament heatingcurrent therefor, and means operatively connected between said first'andsecond circuits to vary the amount of said filament current in aninverse proportion to said positive ion current.

4. A leak detector comprising an evacuated envelope, an exhaust manifoldadapted to be maintained at a low gas pressure, a tubular memberconnecting said manifold in communication with said envelope, one end ofsaid tubular member enclosed within saidenvelope to form an anodeelectrode, a cathode electrode spaced within said envelope from saidanode electrode to maintain a hydrogen gas ionizing electron 1'1discharge therebetween, a. metal sheet permeable to hydrogen when heatedseal .across said enclosed end of said tubular member and in the path ofsaid discharge, means including a negatively charged collector electrodewithin said envelope for establishing a current flow responsive topositive hydrogen ions within said envelope, and means responsive tosaid positive hydrogen ion current flow to maintain a quantity ofhydrogen gas within said manifold.

5. An ionization gauge for detecting a leak in an evacuated system, saidgauge comprising an evacuated envelope. an anode and a cathode electrodespaced within said envelope to support an electron dischargetherebetween, a collector electrode mounted within said envelopeadjacent to the path of the electron dischar e, said anode electrodeincluding a tube extending into the envelope, a palladium closure membersealed across to close the end of said tube within said envelope andarranged to be heated by said electron discharge whereby said closuremember will become permeable to hydro en, the other end of said tubeextending through said envelope to ex ose one face of said member to theexterior of said envelope, said other end of said tube adapted to besealed in communication with said evacuated system.

6. An ionization gau e comprising an evacuated enve ope. a cathode and acol ector e ect ode spaced wi hin said envelope. an anode electrodeincluding a tubular member sealed through said envelope with one end encosed within said envelope and sp c d from s id cathode to sup ort anelectron discharge therebetween, a sheet of metal permeable to hydrogenwhen heated sealed across to close the end of said tubular member withinsaid envelope. said metal sheet arranged to be heated by the dischargebetween said anode and cathode electrodes.

'7. An ionization gauge comprising an evacuated envelope, an anode, acathode and a collec tor electrode enc osed in spaced relationshipwithin said envelope. a portion of said anode within s id envelopeincluding a metal sheet permeable to hydrogen when heated. said metalsheet exposed to the exterior of said envelope and arran ed to be heat dby the discharge between said anode and cathode electrodes.

8, An ionization gauge comprising an evacuated envelope, a plura ity ofe ectrodes including an anode a cathode and a collector electrodeenclosed within said envelo e, said anode including a metal sheetpermeable to hydrogen when heated, one face of said metal sheet exposedto the exterior of the envelope.

9. A leak detector for a vacuum system including a conduit adapted to beconnected to a chamber to be evacuated, an ionization gauge coupled tosaid conduit to be responsive to the conditions of vacuum in saidconduit, a hydrogen permeable barrier between said conduit and saidionization gauge, means for generating a hydrogen gas within saidconduit and means coupled between said ionization gauge and saidhydrogen generating means and responsive to the condition of vacuum insaid conduit for controlling the generation of said hydrogen gas.

10. A leak detector for a vacuum system comprising an exhaust manifoldadapted to be connected to an exhaust pump, an ionization gauge coupledin communication with said manifold, a hydrogen permeable barrierseparating Said ionization gauge and said manifold, said gauge in-'cludingmeans responsive to the presence of hydrogen within saidmanifold, means for generating hydrogen gas within said manifold, andmeans responsive to the change in pressure of the hydrogen within saidmanifold for controlling the hydrogen generating means.

11. A leak detector for a vacuum system comprising an exhaust conduitadapted to be connected to a vacuum pump, an evacuated ionization tubeincluding an anode and a cathode electrode to provide a gas ionizingdischarge therebetween, said anode electrode including a hollow elementforming a communication with said exhaust conduit and an anode portionclosing said communication, said anode portion being permeable tohydrogen at a predetermined temperature, means within said tube formaintaining said anode portion at said predetermined temperature topermit the entry of hydrogen from said conduit into said ionizationgauge, means connected to said tube for indicating the amount ofpositive gas ions within said tube, means for supplying hydrogen to saidconduit including a filament, an electrical circuit therefor and a tubewithin said circult to control the current through said filament,connections between said control tube and said positive ion indicatingmeans for operating said control tube in response to the concentrationof hydrogen within said conduit.

12. A leak detector for a vacuum system including a conduit adapted tobe connected to a vacuum pump, an ionization gauge having a hollow anodecommunicating with said conduit and being permeable to hydrogen underpredetermined temperature conditions, and means within said tube fordetermining the temperature of said anode, an indicating systemconnected to said tube, a hydrogen generating means communicating withsaid conduit and including a filament for generating a hydrogenatmosphere, a heating system for said filament including a control tubeand connections between said control tube and said indicating system forcontrolling the generation of hydrogen in said conduit, said indicatingsystem including an amplifier tube having an output anode, and anindicating device, electrical connections between said amplifier tubeand said indicating device including a load resistance, said controltube including a cathode, anode and control grid, a connection from saidcontrol grid to one side of said load resistance, and an electricalconnection from said cathode to said other side of said load resistancefor determining the bias on said control grid to determine operation ofsaid control tube.

' HERBERT NELSON.

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